I was surfing about the other day, and saw a site referring to the current twin-cam engines as "fatheads"

I don't know if this is some sort of difference between British English and US English? Does "fathead" mean the same thing over there?

come to that, how does this term originate? Flatheads, yes, widely used term which has been around a long while and widely used outside the Harley lexicon. Knucklehead, Panhead, Shovelhead, well-known and generally accepted terms current then and since.

"Evo" seems generally accepted and to some extent, "blockhead" - although I've no recollection of seing that one outside the "custom bike" context and by no means universally applied there either. "Twinkie" or "Twink" seems to be another term occasionally used, although probably not outside the US for the same reason that Terry Pratchett's joke about "Djelibeybi" doesn't work in the US

but "fathead"? Oh, surely not...

Shoot, a man could have a good weekend in Dallas with all that stuff...

"Djelibeybi" is a placename which forms a multiple pun in the "Discworld" books. Basically it doesn't work in the US because jelly babies ( a popular sweet in the UK, Australia and South Africa ) aren't known there by that name. If you have the sort of 1960s classical education which a fair number of his readers probably have, it's a double pun because the place is a parody of Ancient Egypt, based around the "River Djel" and everyone with O Level Latin remembers ( if they remember nothing else ) that "Egypt is the gift of the Nile"

Pratchett subsequently introduced a similar punning placename "Hersheba" which doesn't work in the UK, because we don't have Hershey bars by that name.

likewise "twinks" or "twinkies" for twin-cams doesn't work because we don't have those either.

Shoot, a man could have a good weekend in Dallas with all that stuff...